This invention relates to semiconductor processing and more particularly to a method of improving the crystalline perfection of a layer of monocrystalline silicon epitaxially deposited on an insulating substrate.
It has long been known that the semiconducting properties of an epitaxially deposited layer of silicon in both MOS and bipolar device structures, fabricated on an insulative substrate, is closely related to both the chemical and crystallographic nature of the silicon-substrate interface. The nature of the interface is a function of both the condition of the single crystalline nature of the substrate exposed to the deposition atmosphere and the variables involved in the heteroepitaxial growth process of the silicon film. The properties specific to the thin silicon film grown on the insulative substrate are largely determined by the contamination of the silicon film as a result of reactions with the insulative substrate which take place during the period immediately prior to the complete coverage of the substrate surface.
Detailed examinations of, for example, (100) silicon deposited on a (0112) sapphire will show the presence of defects such as stacking faults and microtwins all of which contribute to increased leakage currents and decreased mobilities in the silicon film in its final, fabricated form. One attempt to improve the deposition technique and to minimize any auto doping of the silicon film at the silicon-insulating substrate interface has been detailed in U.S. Pat. No. 3,885,061, which issued on May 20, 1975 to J. F. Corboy et al. and entitled "Dual Growth Rate Method of Depositing Epitaxial Crystalline Lines." This reference teaches the deposition of a film in two stages, the first stage being the deposition of a very thin film (500-2000 Angstroms) using a "burst" technique followed by the deposition of the remainder of the film at a slower rate until the desired thickness is reached. While the process of this reference does, in fact, produce fewer defects and faults in the deposited silicon layer, it does not address itself to or attempt to cure those defects or faults that are generated in the growth islands when the initial burst produces the individual growth islands.